An electrophotographic image forming apparatus generally has an advantage that it is able to easily form an image of good quality. Thus, electrophotographic image forming apparatus has wide applications including, for example, copying machines, printers, facsimile machines, and complex machines.
Such an image forming apparatus forms an image on a recording material through a charging step of charging the surface of a photoreceptor; an exposure step of exposing the charged surface of the photoreceptor correspondingly to image data so as to form an electrostatic latent image; a developing step of developing the electrostatic latent image with use of toner; a transfer step of transferring the developed toner image to a recording material; and a fixing step of fixing to the recording material the toner image transferred to the recording material.
Developers used in such an electrophotographic image forming apparatus are: single component developers containing only toner; and two-component developers having toner and carrier mixed therein. In general, the two-component developers are frequently used, because they are capable of forming a high-resolution image.
In the image forming apparatus using the two-component developers, the toner and the carrier are stirred in a developer container so as to cause the toner to be triboelectrically charged. The triboelectrically charged toner is adhered to a rotative developing roller and carried to a surface of the developing roller facing an image bearing member, so that the electrostatic latent image on the image bearing member is developed by the toner. Furthermore, the toner density in the developer container is measured. If the toner is insufficient, the developer container is supplied with the toner. In this way, the toner density in the developer container is maintained at a constant level.
For example, Patent Literature 1 discloses the arrangement in which a spike cutting plate (doctor blade) is provided at a brim of the developer container so as to face the developing roller. Such an arrangement aims to reduce difference between the result of the measurement of magnet permeability of the developer and the actual toner density. The difference is caused by variations in density of the developer as a whole. The spike cutting plate causes the developer absorbed by the developing roller to have a uniform height that is a distance from the surface of the developing roller. Here, the distance (doctor gap) between the spike cutting plate and the developing roller is made variable. Further, Patent Literature 1 discloses the arrangement in which two screws are disposed in parallel with the developing roller for stirring and circulating the developer in the developer container. A toner density sensor is provided close to one of the screws that is disposed farther from the developing roller than the other.